gitpoller does not pass environment to git commands

Description

The problem is that gitpoller does not pass the environment to git commands. This is problematic when you have a git repository that is accessed over ssh using an ssh key with a passphrase and you use ssh-agent to cache the keys. In this case, gitpoller does not pass the SSH_AGENT_PID and SSH_AUTH_SOCK environment variables so the git commands to pull down the repo will fail with errors like:

Failure: twisted.internet.utils._UnexpectedErrorOutput: got stderr:
"fatal: ambiguous argument 'master..origin/master': unknown revision
or path not in the working tree.
Use '--' to separate paths from revisions
"

I modified my buildbot locally to fix this issue and will send a GitHub? pull request momentarily.